Field of the Invention
The present invention relates to a marine seismic survey of an underground formation, in particular to an apparatus and method for cleaning an ocean bottom cable comprising seismic node casings and seismic sensor capsules.
Prior and Related Art
As used herein, a marine seismic survey is performed to map an underground formation below a body of water. In the following, the terms “sea” and “ocean” both refer to the body of water and “seafloor” and “ocean bottom” both refer to the interface between an underground formation and the body of water. No distinctions between “sea” and “ocean” should be inferred.
The marine seismic survey is performed by setting off an acoustic source in a series of shots at known positions. Echoes reflected or refracted by rock layers in the underground formation are recorded and analyzed to reveal the depths and elastic properties of the layers, for example in order to identify pockets of porous rock that may contain hydrocarbons before expensive drilling operations are performed.
The echoes may be detected by hydrophones towed behind a survey vessel or by seismic nodes on the seafloor. The latter method provides more information at a higher price, as the seismic nodes contain geophones that detect shear waves (S-waves) in addition to the pressure waves (P-waves) detected by the hydrophones. However, lower prices and increased performance of sensors, electronics and batteries make seismic nodes at the seafloor an ever more affordable alternative to the streamers, especially when the added information provided by the seismic sensors is considered.
An SSR or node may be connected to a cable commonly known as an Ocean Bottom Cable (OBC). The OBC may comprise a data communication line for real time transmission of data to the survey vessel. Alternatively, the OBC may comprise a simple wire or rope provided mainly to facilitate retrieval of the nodes. In this case, autonomous nodes record and store the information for later analysis. Lower prices and increased performance of data storage devices have increased the use of autonomous nodes or Seafloor Seismic Recorders (SSRs) that remain on the seafloor during a the series of shots.
FIG. 1 illustrates deployment of an OBC 19. In particular, a survey vessel 18 on a sea surface 17 moves in a direction 22. A storage reel 21 on the vessel 18 pays out the OBC 19 in the direction indicated by arrow 24 into the sea 30. Once deployed on the ocean bottom 16, the OBC 19 records the echoes from the rock layers in the subterranean formation.
FIG. 2 illustrates retrieval of the OBC 19. It is noted that the vessel 18 moves on the surface 17 in a direction 23 over the OBC 19 such that the nodes are pulled in a substantially vertical direction from the ocean bottom 16, i.e. such that the horizontal forces acting on the nodes (not shown) are minimized. For example, U.S. Pat. No. 6,082,710 (Odim, 1996) col. 2 line 19 states: “Moreover, the cable must run straight up from the ocean bottom, so that there is no tension in any direction along the ocean bottom, as this could cause the cable to become caught on objects on the bottom”. The patent discloses a technique to attain this aim.
Fishing with a longline involves deployment (line setting) and retrieval (hauling) of a line or cable at sea. For example, U.S. Pat. No. 4,920,680 issued May 1, 1990 discloses an apparatus for line setting that includes various rollers and a drive circuit to control the tension in the longline. The document also describes coupling and decoupling ganglions and buoy lines to the longline. Thus, the skilled person searching for solutions in the present field is well advised to search prior art in the field of fishing with longlines for comparable solutions to problems on the present field of technology.
U.S. Pat. No. 5,624,207 A discloses a pulling arrangement for an ocean bottom cable in which friction between the OBC and a series of underinflated tyres is utilized to retrieve the OBC from a seafloor. Other relevant prior art documents are U.S. Pat. No. 5,655,753, in particular FIGS. 3 and 4, and U.S. Pat. No. 5,488,920. For simplicity, numerous details known from the above documents and references cited therein are omitted from FIG. 2, which merely shows the OBC 19 being reeled up in the direction 25 to the reel 21.
WO2010025283 discloses a system for handling a seismic cable with attachable and detachable nodes. The document describes a work station where nodes are attached manually and mentions that the operations may be automated. However, no specifications are given for the automation.
A modern OBC 19 may be up to 20 km long and comprise a node every 25 or 50 meters along its length. Cables are typically deployed 300-21 meters apart to form an array recording the echoes from shots fired at predetermined spots over the array. In order to store the required amount of cables, and to handle the number of nodes involved efficiently, a cable comprising removable sensor capsules has been proposed. This cable is illustrated on FIG. 3 and will be explained below. Further, NO 20121418 A1 describes an apparatus and methods to load and unload sensor capsules automatically from node casings forming part of the cable.
U.S. Pat. No. 6,070,857 discloses use of conveyors for use in the field of the present invention. As for the type of conveyor, there are at least three types in common use: containers pulled by an endless chain or rope, conveying belts and conveyors comprising numerous rotating rollers disposed side by side. These and other types are considered well known to one skilled in the art, and will not be discussed in greater detail as such. However, it should be noted that conveyors wherein an object is attached to or detached from a line running at constant speed are known from the field of line setting and hauling mentioned above, and also from endless rope conveyors such as gondolas or skilifts. Thus, when searching for solutions to problems regarding attaching or detaching an object from a constantly running seismic cable, the field of rope conveyors in general and skilifts in particular may contain viable prior art solutions.
FIG. 3 illustrates a seismic cable 19 of a kind used in the present invention. The cable comprises autonomous seismic nodes 29 interconnected by stress elements 4, in this embodiment formed by a steel wire. Each seismic node 29 comprises a node casing 5 and a removable sensor capsule 9 located in an inner space 15 of the node casing. The left hand side of FIG. 3 shows the sensor capsule 9 outside the inner space 15, while the right side of FIG. 3 shows the sensor capsule 9 inside the inner space 15. Each sensor capsule may comprise one or more not illustrated geophones, hydrophones, accelerometers, processors for executing program code, clocks, memories, movement sensors, temperature sensors, input/output means, power supplies, e.g. batteries, internal communication means and other components necessary to measure, record and store seismic signals, and possibly also perform some initial signal processing before the data are stored. Acoustic decoupling arrangements 6 between the node casings 5 and the stress elements 4 stop or reduce propagation of acoustic signals and noise between the seismic nodes 29.
Before deployment, the node casings 5 are stored on one or more reels 21 as part of the seismic cable 19, and the sensor capsules 9 are stored in a suitable storage. The sensor capsules 9 are preferably loaded automatically into the node casings 5 on board the vessel 18 during deployment, e.g. by the loading/unloading apparatus disclosed in Norwegian patent application NO 20121418 A1.
During retrieval, the sensor capsules 9 are preferably unloaded automatically from the node casings 5, e.g. by the loading/unloading apparatus disclosed in NO 20121418 A1. Then the sensor capsules are brought to one or more service stations, where the recorded data are retrieved. The service station(s) may also be responsible for replacing or recharging batteries, reprogramming of processors, synchronizing and recalibrating clocks and other tasks during retrieval and/or immediately before redeployment.
In order to handle a large number of nodes efficiently and to protect the service station(s), the sensor capsules should preferably be clean, free from salt and dry when they are put into the station after retrieval of the cable 19. Similarly, the other parts of the cable 19, e.g. the steel wire 4 and flexible node casings 5 should be reasonably clean, possibly free from salt or even dry to avoid corrosion and unpleasant odors when the cable is stored on the reel 21.
In principle, an OBC and/or SSRs may be deployed in shallow waters and be subject to fouling, i.e. growth of barnacles, weed and/or other marine organisms. Fouling may be removed by passing the cable through a ring shaped flushing tool suspended outside the vessel. This tool uses pressurized seawater to flush any fouling into the sea before the cable enters the vessel. “Fjerning av marin begroing” (“Removal of marine fouling”) by Gloppen et. al, HSH, 2009 provides an overview of techniques and discloses a ring shaped flushing tool using pressurized seawater for removing marine fouling. Of course, such a flushing tool could also be employed to flush sand, clay and grit back into the sea.
It is understood that fouling is not a great concern at the depths and times involved in a typical survey as discussed herein, mainly because light does not penetrate to the depths of concern. When a node travels through a hundred meters or more of water between the seafloor and the vessel during retrieval, most particles, e.g. sand and grit, on the surface of a node casing fall off. However, particles lodged around the sensor capsule within the node casing may still be present at the surface during retrieval.
A first objective of the present invention is to provide an apparatus and a method to ensure that the components of the ocean bottom cable are reasonably clean, and sufficiently free from salt and dry when it is reeled up on a reel during retrieval. A second objective is to achieve the above objective in a cost efficient and environmentally sound manner.